Cavity backed spiral antenna



' n A ril 29, 1969 D. N.CLASB'Y ETAL: r 3,441,937

' CAVITY BACKED SPIRAL ANTENNA Filed Sept. 28, 1967 IN VE'N TORS DA V/D M CZASBV R0 ERA/ ALL BY Arrokusy United States Patent 3,441,937 CAVITY BACKED SPIRAL ANTENNA David N. Clasby, Newhall, and Roger D. Hall, Encino,

'Calif., assignors to The Bendix Corporation, a corporation of Delaware Filed Sept. 28, 1967, Ser. No. 671,380 Int. Cl. H01q 1/36 U.S. Cl. 343-895 7 Claims ABSTRACT OF THE DISCLOSURE Background of the invention The invention herein is a spiral type microwave antenna including means for minimizing cancellation of the radiation from the back side. Where operation in the gigahertz frequency range is desired, the antenna necessarily becomes quite small, and it has proved to be quite difficult to arrive at a configuration having small size and which has a wide bandwidth and yet has a good pattern over the bandwidth. It freqeuntly happens that operation is acceptable over part of the band, but at some frequencies within the band a cancellation occurs due to reflections from the bottom of the cavity. A significant advantage of the structure described herein is that it minimizes these cancellation eifects, thus retaining a good pattern over a wide band.

Summary of the invention The specific antenna design taught herein was developed after a considerable eifort directed toward attempting to accomplish the desired performance within the framework of minimum size and weight. It is particularly useful for installation in aircraft, since a typical antenna operable in the 4 GHz frequency range is less than three inches in diameter and weighs less than onehalf pound. Through the combination of the specific cavity design including the liner of resonant microwave-ab sorbent material, the plastic foam filler material and the spiral printed circuit antenna board supported by the filler material, a highly efiicient small broadband antenna has been produced. This configuration is also effective in coping with environmental problems resulting from the heat, vibration and pressure changes experienced in airborne use. p

The printed circuit antenna actually includes two separate spiral elements which are insulated from each other and which are connected near the center of the board to connecting means which in one embodiment is a coaxial cable and which in the other embodiment is a pair of elements in a waveguide which is then connected to a coaxial cable. The use of the waveguide adds parts and expense, but produces higher performance through better balance between the spiral elements.

Description of the drawings FIGURE 1 is an exploded view of an antenna incorporating our invention;

3,441,937 Patented Apr. 29, 1969 FIGURE 2 is a sectional view of a modified form of antenna incorporating our invention; and

FIGURE 3 is a sectional view taken along line 3-3 of FIGURE 2.

Description of the preferred embodiments Referring now to FIGURE 1, the microwave cavity consists of a fiat aluminum cylinder 10 whose height is typically substantially less than its radius and which may be about one-half of its radius. Since it supports the outside edge of the spiral antenna, the diameter of the cavity is largely dictated by the frequency range for which the antenna is designed, as is known by those skilled in the art. Positioned against the inside cylinder surface of the cavity 10 is a circular piece 12 of resonant radio frequency absorber material which is typically a silicon rubber matrix having microwave absorber material dis persed throughout. Such material is commercially available through a number of names and designations. One such material with which applicants are acquainted is known in the industry as Eccosorb SF-ll, manufactured by Emerson and Cumming of Camden, Massachusetts. Similarly, a ring of this material 14 is positioned against the interior side walls of the cavity 10 in such manner that it abuts against the circular member 12. Carried within member 14 is a flat cylindrical member 16 which is typically of a rigid plastic foam, such as polyurethane foam. The properties desired of member 16 are primarily that of sufficient structural strength and rigidity to retain its dimensional stability despite susbtantial changes in temperature and pressure and a dielectric characteristic similar to that of free space or air. Certain materials which would be acceptable from a mechanical standpoint and also from the standpoint of electrical insulation have proved to be unsatisfactory because of a tendency to absorb or adsorb moisture, which adversely affects the dielectric properties. Abutting against the end of members 16 and 14 and the cavity 10 is a circular printed circuit board 18 having two separate spiral conducting tracks thereon which are electrically insulated from each other and which terminate at the center in connectors 20 and 22. A standard coaxial connector member 24 is bolted through a passageway 26 in the bottom of cavity member 10 and is held in position by means of a washer 28 and a nut 30. Members 12 and 16 include passageways 32 and 34, respectively, which contain the connector 24 in assembled position. The terminals 20 and 22 of the spiral antenna board 18 are soldered to connector pins 36 and 38, respectively, of the connector 24. It will be recognized that when the entire assembly is put together, the circuit board will be abutted tightly against the outside surface of member 16 and the edge of cavity 10.

A modified form of our invention is shown in section in FIGURE 2 wherein the cavity 40, which may be very similar to cavity 10, includes a layer of resonant RF- absorbent material 42 on its inner surfaces. This material may be the same as that used for members 12 and 14. Positioned within the absorbent material 42 is a polyurethane foam member 44 which may be idcntical to member 16. A printed circuit board 46 which may be similar to member 18 is fastened tightly against the edges of cavtiy 40 and the outer face of foam member 44. At the back surface of microwave cavity 40 is positioned a waveguide structure 48 which may be attached by any suitable means. Attached to the interior top and bottom surfaces, respectively, of member 48 are waveguide elements 50 and 52, each of which includes an elongated portion which passes through the center of the cavity member 40 and the foam member 44 to make connection with one of the spiral tracks on the face of circuit board 46. This connection may be effected by any suitable means, such as by soldering or through the use of small screws such as those shown at numerals 54 and 56. It will be observed that waveguide elements 50 and 52 are electrically connected with a pair of outside and inside conductor elements 58 and 60, respectively, of a coaxial connector member which is arranged to be threadedly engaged with a coaxial cable (not shown). The waveguide elements 50 and 52 have closely spaced parallel faces 62 and 64 which form the actual waveguide, and the spacing of these faces is determined by the general frequency range of the antenna.

It will be observed that the embodiment of FIGURES 2 and 3 requires more space in the axial dimension than does the FIGURE 1 embodiment, and because of the additional parts which must be manufactured within moderately close tolerances the cost of this embodiment is greater than that of FIGURE 1. The waveguide structure does, however, provide better balance between the two spiral antenna elements and avoids some loss, thereby giving significantly better performance.

While only two embodiments have been shown and described herein, modifications will be apparent to those skilled in the art, and we do not desire to be limited except in accordance with the terms of the following claims.

We claim:

1. A Wide bandwidth microwave antenna comprising:

a cylindrical cavity of electrically conductive material whose height is substantially less than its radius,

a layer of microwave-absorbent material covering the interior surfaces of said cavity,

a cylindrical member of insulating material having dielectric strength approximating that of air substantially filling said cavity within said absorber, said member having an axially located center passageway,

a spiral antenna mounted on a substantially circular member of insulating material positioned on the outside surface of said cylindrical member,

and electrical connecting means passing through the 4 axis of said cavity and said cylindrical member connected to said spiral antenna.

2. A wide band microwave antenna as set forth in claim 1 wherein said cylindrical cavity is of material which is essentially aluminum and waveguide means are attached to the end of said cavity opposite to said antenna.

3. A wide band microwave antenna as set forth in claim 1 wherein said microwave-absorbent material consists of a silicon rubber matrix having ferrous material dispersed therethrough.

4. A wide band microwave antenna as set forth in claim 1 wherein said cylindrical member is of polyurethane foam material.

5. A wide band microwave antenna as set forth in claim 1 wherein said spiral antenna includes a printed circuit board wherein the conductors are flat spiral tracks, said tracks being connected near the center of said board to said electrical connecting means.

6. A wide band microwave antenna as set forth in claim 2 wherein said waveguide means includes a rectangular hollow member fastened to said cavity and said electrical connecting means includes a pair of guide members attached to opposite sides of the interior of said hollow member having flat surfaces closely and uniformly spaced from each other.

7. A wide band microwave antenna as set forth in claim 1 wherein the height of said cylindrical cavity is approximately one-half of its radius.

References Cited UNITED STATES PATENTS 3,192,531 6/1965 Cox et al. 343895 ELI LIEBERMAN, Primary Examiner.

US. Cl. X.R. 343789 M. NUSSBAUM, Assistant Examiner. 

